This invention relates to developers for use in electrophotographic recording equipment, and more specifically, it relates to dry particulate developers ("toners") having magnetic and other properties particularly suited for direct imaging, together with novel processes for producing such developers.
In many electrophotographic processes, a photoconductive surface (usually coated on a substrate) is electrostatically charged, and an image to be reproduced is projected thereon to produce a latent electrostatic image corresponding to the light and dark pattern of the projected image. This latent electrostatic image is then developed by contacting the surface of the photoconductor with a developing agent or "toner" which adheres electrostatically to the imaged areas. In one widely used technique, the application of the toner to the latent image is carried out with a developer mixture composed of carrier beads and toner which adheres to the carrier beads by electrostatic attraction. The carrier bead-toner mixture is distributed over the photoconductive surface through some mechanical action such as brushing, cascading, immersion or the like, and the toner adheres electrostatically to the charged areas. The degree of development is crucial to the clarity and intensity of the image.
After the image has been toned, or made visible on the photoconductive surface, additional steps must be taken to provide a fixed image for later use. Two general systems are prevalent in electrophotography to produce a permanent fixed image: direct systems and transfer systems. In direct systems, the image is developed on the photoconductor, and the developed image is directly bonded to the surface of the photoconductor. In transfer systems, the latent image on a surface is transferred to another substrate, by electrostatic means, and then bonded to the new substrate to produce a permanent image.
The bonding action, in either type of system, has been achieved by several methods. A common method is to apply heat to the substrate and thereby to melt the thermoplastic resin in the toner, which will fuse the pigmented toner to the substrate. Another method is to apply pressure to the toner and substrate, so that resinous material in the toner is forced onto the substrate to form a bond and hold the toner to the substrate. Solvents have been used, partially to dissolve the resins in the toner, creating an adhesive bond between the toner and the substrate.
Other types of developer inks occur, but these are manufactured by admixing pigments and resins by blending, mixing or milling, and, to obtain a uniform distribution of the pigment in the resin, the blending must be done above the melting point of the thermoplastic resin. The mixture is fused and allowed to cool and harden. The solid composition is then broken up or pulverized, as by ball milling. The particles are then treated with additional additives to alter their electrical properties and are further subjected to processing methods which melt or soften the resin to allow for partial or complete mixing of the additives. Powders prepared by this method suffer from a number of disadvantages among which are:
1. Wide particle size distribution: This is due to the grinding or pulverizing operations, and additional steps of particle size classification are generally required to create a useful developer ink.
2. Particles of non-uniform electrostatic properties: This is due to the need to blend components or additives into the hardened blend of resin and pigment, without softening the resin, to a point where uniformity can be obtained.
3. Expensive and tedious material and processing operations, with a low yield of usable material.
The art of electrophotographic recording devices and equipment is well known. Xerography, the most widely practiced form of electrophotography, and the composition of toners, therefor, are more fully described in Carlson U.S. Pat. No. 2,297,692, and Walkup U.S. Pat. Nos. 2,228,416; 2,618,551, and 2,618,552. Brynko U.S. Pat. No. 3,745,118 describes a toner which contains agglomerated pigment particles overcoated through spray drying with a thermoplastic resin.
Other types of electrophotographic devices require toners having differing electrical properties, and have been described in West U.S. Pat. No. 3,166,510, as well as in Seymour U.S. Pat. Nos. 3,196,032 and 3,280,741, and in Nelson U.S. Pat. No. 3,639,345. Magnetic developing techniques have been described in Gundlach U.S. Pat. No. 3,166,432 and in Kotz U.S. Pat. No. 3,909,258.